Much interest has developed in mechanical properties of brain tissue because these properties are believed to play a critical role in studying head injuries or simulations of brain deformation during surgery. It is desirable to know these properties so that not only can head injuries be simulated in computer and mechanical models, but that the effect of ameliorative devices, such as helmets, can be tested on computer and mechanical models rather than on live people. For example, when an object strikes a human head, there are effects on the brain both on the “coup” side, where the object struck, and on the opposite or “contra-coup” side; even if the skull remains intact and the brain is not penetrated, these effects can lead to bruising, swelling, confusion, even bleeding and, in some cases, death. The effects on both coup and contra-coup side of the head depend significantly on the dimensions, mechanical and physical properties of brain tissue and surrounding structures, including the skull, meninges, and cerebrospinal fluid, and how the brain is accelerated by the blow, and decelerated by the opposite side of the skull. Among the mechanical properties of brain on which such effects depend are tissue shear stiffness, including time decay constants between short term stiffness for rapid motions, and long term stiffness for slower motions, bulk modulus, density, and viscoelasticity.
Mechanical properties of brain tissue derived from cadavers may be inaccurate due to rapid degenerative changes that begin at death. Material properties of the brain are still poorly understood and existing material models have the instantaneous shear stiffness differing up to two orders in magnitude. This is largely due to the paucity of in vivo brain biomechanical data and variations in mechanical testing schemes and tissue postmortem conditions. Brain tissue is particularly sensitive to anoxia, hours-old tissue is suspected to respond significantly differently than fresh, living, tissue. It is therefore desirable to measure these properties in-vivo.
Arash A. Sabet, Eftychios Christoforouc, Benjamin Zatlin, Guy M. Genin, Philip V. Bayly, Deformation of the human brain induced by mild angular head acceleration, Journal of Biomechanics 41 (2008) 307-31, available online at www.JBiomech.com, and Y Feng, T. M. Abney, R. J. Okamoto, R. B. Ness, G. M. Genin and P. V. Bayly Relative brain displacement and deformation during constrained mild frontal head impact J. R. Soc. Interface, doi:10.1098/rsif.2010.0210, published online at rsif.royalsocietypublishing.org, describe efforts to use high speed tagged MRI images to measure deformation of the brain under mild acceleration. These high speed tagged MRI images are compared with computer simulations of brain deformation under similar acceleration to validate parameters of those models.
Many people undergo brain surgery each year, often for tumors. Tumors often have mechanical properties significantly different from those of surrounding normal tissue. Measurements of mechanical properties of brain at particular areas near diseased tissue may provide clues to hidden metastases a surgeon may wish to treat.
As part of the surgical process, a craniotomy is generally performed to expose tissue. Once opened, pressure changes due to the craniotomy causes a deformation of brain tissue. The surface of tissue deformed from craniotomy has been mapped with a stereo camera. Models of pre-craniotomy brain shape can be derived from the MRI and CT images available for many patients, and correlated to post-craniotomy brain shape.
The surface of the brain has many convolutions and small blood vessels. Brain surface topography has been mapped by 3-dimensional topography extraction software.